1. Field of the Invention
The invention relates to an antenna device that receives radio waves transmitted from a satellite, and more particularly, to an improvement to a waterproof structure therefor.
2. Description of the Related Art
Conventional systems for directing and guiding the course of an automobile to the driver of the automobile, so-called car navigation systems have come into widespread use. In such a car navigation system, the present position of the automobile is specified based on the speed or traveled distance of the automobile, while the present position is also specified based on positional information obtained from radio waves transmitted and received from GPS satellites in order to improve the positioning accuracy.
In recent years, in the United States of America and other countries, digital radio broadcasting has come to be provided using radio waves transmitted from an artificial satellite. An antenna is necessary in a digital radio receiving system to receive the digital radio broadcasting, and a so-called DAB (Digital Audio Broadcasting) antenna is used.
Radio waves from a satellite are often in a high frequency band and have high directivity. Therefore, in the car navigation system and digital radio receiving system described above, the receiving antenna must be attached to the top surface (such as the roof) of the automobile in order to receive the radio waves from the satellite in a good receiving condition.
Therefore, an antenna device that receives radio waves from a satellite must have high weather and water resistance.
As shown in FIG. 6, a conventional GPS receiving antenna includes an antenna module 100 that receives radio waves transmitted from a GPS satellite, and the antenna module 100 is stored in an internal space formed by a cover member 101. The cover member 101 has a hole 101a on its one side, and a transmission cable 102 leading from the antenna module 100 is externally extended from the hole 101a. A first waterproof packing 103 is attached to the transmission cable 102 in the position of the hole 101a. 
The cover member 101 is provided with a second waterproof packing 104 to seal the open side as the antenna module 100 is stored and then a bottom plate 105 supporting the second waterproof packing 104. The second waterproof packing 104 and the bottom plate 105 are fixed to the cover member 101 by four screws 106.
The bottom plate 105 is provided with a magnet 107 for securing the GPS receiving antenna to the roof of the automobile. At the outer side of the bottom plate 105, a sheet type member 108 of for example PET (polyethylene terephthalate) is adhesively provided in order to hide the heads of screws 106 for improved appearance and prevent the roof of the automobile from being damaged by the bottom plate 105. The sheet type member 108 has a transparent part In the center, and an indicator tag 109 is provided between the sheet and the bottom plate 105. The model number of the GPS receiving antenna and the like in the indicator tag 109 can be recognized through the transparent part of the sheet type member 108. In the conventional GPS receiving antenna described above, the water resistance is secured by the first waterproof packing 103 and the second waterproof packing 104 of silicon rubber or the like, and the antenna module 100 stored in the cover member 101 is protected
As described above, in the conventional GPS antenna, the water resistance is secured by the first waterproof packing 103 and the second waterproof packing 104. The bottom plate 105 and the four screws 106 are provided to support and fix the second waterproof packing 104 (see Japanese Patent Laid-Open No. 2001-68912).
In this way, the conventional GPS antenna requires a large number of parts and there is a limit to the reduction of the parts and the assembly cost, and it is difficult to reduce the overall cost.